The present invention relates to a method which permits the resolution of a monopulse radar to be improved by processing the various signals which are obtained, in the conventional manner, in a radar of this type. The invention likewise relates to an arrangement which can be assigned to a monopulse radar in order to implement this method.
It is known that, with the aid of two receivers which are arranged in a manner such that their axes of reception are slightly out of alignment with the axis of transmission, a monopulse radar is capable of providing two separate signals, one of which corresponds to the sum of the signals received by the two receivers and gives the same indications as a conventional radar, while the other signal corresponds to the difference of these two signals and gives a difference-signal which enables the error-angle between the direction of the detected target and the antenna axis to be measured.
When the detected target is complex and is composed of several reflecting zones, known as bright points, either because of its shape or because of the fact that it is composed of several separate objects, the signals delivered by the monopulse radar are observed to be subject to significant fluctuations. These fluctuations, known as glint, are due to interference effects between the waves, which are reflected by the various bright points, and are capable of giving rise to incorrect indications or even grossly misleading indications.
Two distinct types of arrangement have been used in order to avoid this phenomenon.
The first type of arrangement, corresponding for example to French Patent 2,396,311, in the name of the Applicant Company, is known as an antiglint device. It utilizes the strong correlations between the sum channel, a conventional angle-error channel and an additional angle-error channel, which is called quadrature angle-error channel and corresponds to the imaginary part of the complex difference-signal, the real part of the latter corresponding to the conventional angle-error channel.
These correlations enables an effective adaptive-filtering procedure to be set up, which enables a direction to be determined, corresponding in fact to the center of mass of the bright points of the target.
When this target is on its own, this center of mass is, as a rule, situated inside the target, this fact justifying the interest in antiglint devices of this nature, of which numerous embodiments are known.
However, no further information are available regarding the dimensions of the detected target, which would be particularly important if the target is composed of several separate objects, such as aircrafts making up a squadron. The ultimate objective is to open fire at these aircrafts; but in this case there is every chance of firing at empty space: For example a missile which is guided by a device of this type will pass between two aircrafts, which are detected simultaneously, without touching either of them.
The second type of arrangement utilizes the fact that the bright points of a target approach or move off with a velocity relative to the radar antenna, corresponding to a Doppler effect which is particular to each bright point. The velocity of each bright point is associated with its position in the target, and the analysis of the Doppler spectrum from the whole of the target may therefore make it possible, at least in theory, to measure the velocity of each of the bright points and thus to reconstitute the overall dimensions of the target.
In order to obtain this result, it is nevertheless necessary to carry out a highly discriminatory spectral resolution procedure on the Doppler spectrum of the received signal, this procedure on the one hand being very difficult and on the other hand necessitating a very large number of calculations which have to be performed very rapidly: This second method has not given rise to satisfactory embodiments.
the invention proposes to process the fluctuations in the angle-error signals in order to extract from then the information corresponding to the external dimensions of the target, without seeking solely to reduce these fluctuations, as in the case of the conventional antiglint devices. For that purpose, a model of the target is produced, by reducing it to the two principal bright points at the greatest distances from its center of mass. By then processing simultaneously the sum channel, the conventional angle-error channel and the quadrature angle-error channel between two consecutive extreme values of the sum channel, a measurement of the wingspan of the target is obtained together with a measurement of the directions of the two bright points representing the target.